For all the chatter about Facebook and Twitter, it's a huge statement for Pinterest to be just one percent behind Twitter in terms of users (with a margin of error of +/- 2.6 percentage points). It's perhaps just as big a statement for Facebook to have such a dominant lead on Twitter, even though Twitter continues to get plenty of love in news circles and at the Super Bowl.

For Pinterest, the 15 percent user number is up from 12 percent in August 2012, suggesting it continues to grow. Instagram placed fourth in the study with 13 percent of American Internet users on the service.

The site has historically skewed female in terms of users. Eight of the top 10 pinners in terms of followers are women, per Repinly. Women outnumber men on Pinterest by 5-to-1, Poynter reports. The service is most appealing to "women, adults under 50," the Pew study said.

Notably not mentioned in the Pew report is Google+, which said it was the second most popular social network just last month, based on a study of active users per social network. That said, other studies and likely time-on-site numbers aren't in Google+'s favor. Unlike other social networks, it has the advantage of boosting its numbers with its popular search engine.